Process for the preparation of printing plates



United States Patent 3,309,990 PROCESS FOR THE PREPARATION OF PRlNTlNGPLATES Kurt-Walter Kliipfel and Martha Tomanek, Wiesbaden- Biebrich, andFritz Endermann, Wiesbaden, Germany, assignors, by mesne assignments, toAzoplate Corporation, Murray Hill, N5. N0 Drawing. Filed Dec. 26, 1961,Ser. No. 162,278 Claims priority, application Germany, Jan. 25, 1961,

Claims. ((311. 101-1492) It is known, to produce copiesele-ctrophotograiphically as follows: an electrophotographic material iselectrically charged and then exposed to light, either by the contactprocess, in which the light passes through the master, or by episcopicprojection; the resultant latent electrostatic image is made visible bypowdering over with a pigmented resin powder and then fixed, for exampleby heating, whereupon the pigmented resin powder becomes irremovablyanchored to the support. The electrophotographic material used for thisprocess consists of a support and a photoelectrically conductiveinsulator coating applied thereto, the latter consisting of orcontaining a photoelectrically conductive substance.

From copies of this type, obtained by the electrophotographic process,printing plates have been prepared by treatment with a solvent. Thecoating is removed by the solvent from the support in the image-freeareas and, if necessary, the bared parts are rendered water-conductiveby suitable means; the image parts are then inked up with greasy ink.Printing plates for the production of prints are thereby obtained. Thisknown process has the disadvantage that to a considerable extent thesolvent dissolves away the image-bearing parts of the reproductioncoating also, so that printing plates with but inadequate properties areobtained.

Also, lithographic printing plates have been prepared by the applicationto a support of a coating of a hygroscopic substance which, in turn, iscovered by a photoconductor coating. In this process, the disadvantageis that the hygroscopic coating must be applied first and thephotoconductor coating afterwards. Furthermore, when the printing platesare being prepared, the hygroscopic coating must be bared in theimage-free parts. This coating tends, however, to retain fine particlesof the photoconductor coating that is being dissolved away, so thatfrequently, when printing is performed, prints are obtained which arenot free of background, i.e., the prints have traces of ink in theseparts.

For the preparation of a water-conductive surface in the image-freeparts of a photoelectrically conductive inorganic insulator coating,water-soluble salts of bivalent metals from Groups I, II, IV, VII andVIII of the periodic system have been proposed, e.g., zinc acetate in anaqueous alcoholic solution, and a mixture of zirconyl acetate and sodiumacetate in butanol, formaldehyde and water. However, images completelyfree of background are still not obtained as a result of treatment withthese salt solutions.

Solutions of hexacyano-iron acids to which acetic acid, tartaric acid orcitric acid has been added have also been disclosed for the conversionof an electrophotographic copy into a printing plate. Coatings treatedin this way have, however, the disadvantage that they tend to scum aftera few prints have been made and in general give ice only a shortprinting run as the effect of this mixture is not specific.

The present invention relates to a process for the preparation ofprinting plates in which the image-free parts of a photoelectricallyconductive insulator layer containing inorganic photosemiconductors, onwhich an image has been produced by electrophotographic methods, aremade hydrophilic by treatment with an aqueous solution containing one ormore salts of the hexacyano-iron (II)- acid and/or the hexacyano-iron(III)-acid and one or more organic acids of high molecular weight.

The process of the invention has the advantage over the processeshitherto known that it yields printing images that are completely freeof background. Considerably longer runs than those hitherto possible canbe obtained, no greasy ink being taken up by the image-free parts in anycase. With this treatment, the image parts, which are covered with resinpowder, remain water-repellent and can be inked up with greasy ink. Theprocess by which the parts are rendered water-accepting with thesolution of the invention is performed very easily and very quickly, sothat an electrophotographic copy needs only to be set up in an offsetprinting machine, wiped over with the solution, and in this way madeready for printmg.

As inorganic compounds which can be used as photosemiconductors in thepresent process, the following are mentioned by way of example: thephotoconductive oxides, sulfides, selenides, tellurides and iodides ofzinc, bismuth, molybdenum, lead, antimony and cadmium. Also, mixtures ofa number of the photosemiconductors mentioned can be employed; further,mixtures of organic photosemiconductors with the inorganicphotosemiconductors mentioned above are suitable.

As organic photoelectrically conductive compounds the following areexemplary: oxadiazoles, e.g., 2,5-bis-(4- diethylaminophenyl-( 1))-l,3,4-oxadiazole, 2,5-bis (4' (n-propylamino)-2'-chlorphenyl-(1))-1,3,4-oxadiazole or 2,5 bis (4 N-ethyl-N-n-propylaminophenyl-(1'))-1,3, 4-oxadiazole; triaz-oles, e.g., 1 methyl 2,5 bis(4-diethylamino-phenyl (1')) 1,3,4-triazole; imidazoles, e.g., 2-(4-dimethylaminophenyl) -6-methoxy-benzimidazole; oxazoles, e.g.,2-(4-chlorphenyl)-phenanthreno-(9,1 O:4,5) oxazole; thiazoles, e.g.,2-(4-diethylaminophenyl)-benzthiazole; thiophens, e.g.,2,3,5-triphenyl-thiophen; triazines, e.g.,3-(4'-aminophenyl)-5,'6-di-pyridyl-(2)-1,2,4- triazine or3-(4'-dimethylaminophenyl)-5,6-di-(4-phenoxyphenyl)-1,2,4-triazine;hydrazones, e.g., 4-dimethylaminobenzaldehyde-isonicotinic acidhydrazone; styryl compounds, e.g.,2-(4-dimethylaminostyryl)-6-methy1-4-pyridone or2(4-dirnethylaminostyryl)-6-methyl-4-pyrone or 2-(4-dimethylaminostyryl)-5-( or 6) -amino-benzimidazole; azomethines, e.g.,4-dimethylaminobenzylidene-{imaphthylamine; amino compounds, e.g.,3-dimethylamino-1,2- benzophenazi-ne; and polyvinylcarbazole.

The photosemiconductors can the applied in known manner to a suitablesupport as a suspension in .a solution containing a natural or syntheticresin, e.g., silicone resin, acrylic resins, ketone resins, balsamresins, phenol resins modified with colophony, cournarone and indeneresins, as Well as processed natural substances such as celluloseethers, polymers such as polyvinyl chlorides, polyvinyl acetates,polyvinyl alcohols, also polystyrene and isobutylene, polycondensates,such as phthalate resins, alkyd resins, maleinate resins,phenol-formaldehyde resins and polyadducts, e.g., polyurethanes.

To increase the light-sensitivity of the reproduction coating in thevisible region of the spectrum, sensitizers are added in smallquantities (about 0.001 to 0.5 percent by weight with respect to thequantity of photoconductor employed in the reproduction coating) to thesolutions in which the photoconductor is suspended or they are appliedby subsequent treatment after the electrocopying material has beenprepared. As sensitizers, dyestuffs, the identification number of whichis given below under which they are listed in the SchultzFarbstofftabellen (7th edition, 1st vol., 1931), are particularlysuitable. Examples of effective sensitizers are: triaryl methanedyestuffs such as Brilliant Green (No. 760, p. 314), Victoria Blue B(No. 822, p. 347), Methyl Violet (No. 783, p. 327), Ethyl Violet (No.787, p. 331), Crystal Violet (No. 785, p. 329), Acid Violet 63 (No. 831,p. 351); xanthene dyestuffs, more particularly Rhodamines such asRhodamine B (No. 864, p. 365), Rhodamine 6G (No. 866, p. 366), RhodamineG Extra (No. 865, p. 366), Sulphorhodamine B (No. 863, p. 364) and FastAcid Eosin G (No. 870, p. 368), as also phthaleins such as Eosin S (No.883, p. 376) Eosin A (No. 881, p. 374), Erythrosin (No. 886, p. 376),Phloxin (No. 890, p. 378), Bengal Rose (No. 889, p. 378) and Fluorescein(No. 880, p. 373); thiazine dyestuffs such as Methylene Blue (No. 1038,p. 449); acridine dyestuffs such as Acridine Yellow (No. 901, p. 383),Acridine Orange (No. 908, p. 387) and Trypofiavine (No. 906, p. 386);quinoline dyestuffs such as pinacyanol (No. 924, p. 396) andCryptocyanine (No. 927, p. 397); quinone dyestuffs and ketone dyestuffssuch as Alizarin (No. 1141, p. 499), Alizarin Red S (No. 1145, p. 502)and Quinizarine (No. 1148, p. 504); cyanine dyestuffs, e.g., cyanine(No. 921, p. 394) and chlorophyll.

As the supports for the electrocopying layers, foils made of metals suchas aluminum, zinc, copper and brass are preferably employed. Othersupports used in electrophotography such as paper, or foils made ofpaper or plastic with vacuum deposited or laminated metal surfaces canalso be used. The papers may, moreover, be provided with a conductiveprecoat which is resistant to organic solvents, e.g., like thatdescribed in U.S. Patents 2,534,650, 2,681,617 or 2,559,610.

In applying the reproduction coatings to the supporting material, thephotoconductive substances are suspended in a resin solution by aprocess of homogenization in a colloid mill and then applied to thesupport in a thickness of 20-40 grams/square meter, preferably 2835grams/ square meter. This can be done in known manner by spraying,painting, roller application, immersion in the suspension, orapplication of the suspension to the rotating supporting material. Thecoating is then advantageously dried at an elevated temperature to driveoff residual solvent.

The preparation of the electrocopies is in known manner, i.e., theelectrocopying material described above is charged with a chargingdevice, by means of a corona discharge, and then exposed imagewise bythe contact process, the light passing through the master. The materialcan also be exposed to an episcopic projection or it can be exposeddirectly in a camera. The latent electrostatic image is then developedin known manner with one of the usual pigmented resin powders, which mayin some cases be suspended in a dielectric liquid and the image thatbecomes visible is fixed, e.g., by heating with an infra-red radiator toabout 100-470, preferably 130-160" C., or by treatment with solvents, sothat irremovable images are obtained which have good contrast.

In converting the images obtained by the electrophotographic processabove described into printing plates, these are contacted with asolution containing water and one or more salts of the hexacyano-ironacid and one or more organic acids of high molecular weight.

The substances included in the term salts of the hexacyano-iron acidsare salts which preferably contain as the cationic component alkalissuch as sodium and potassium,

but alkaline earths such as calcium and magnesium or other cations canalso be used.

As the hexacyano-iron acids of the invention, hexacyano-iron (II) acid,which is represented by the formula H [Fe(CN) and hexacyano-iron (III)acid, which is represented by the formula H [Fe(CN) are contemplated.

Also, mixtures consisting of the salt-forming metals listed above andthe two hexacyano-iron acids can be used.

Among the organic acids of high molecular weight which can be used inaccordance with the invention, the following are preferable:polymerization products of phosphonic acids which are substituted withorganic polymerized residues, e.g., polyvinyl phosphonic acid; moreoverpolymerization products of substitted or unsubstituted acrylic acids,e.g., polyacrylic acid and polymethacrylic acid are suitable.

The treatment of the image for the purpose of conversion into a printingplate may consist, for example, simply of a wiping over process, e.g.,with a wetted cotton pad, or immersion of the foil in the solutionimparting hydrophilic properties; the foil may even be first set up inan offset machine and the surface then wiped over with a wetted spongecontaining the solution. The image-free parts immediately becomeWater-conductive so that printing may be immediately effected from thefoil. It is advantageous for the printing plate to be Washed down withwater after the treatment.

Aqueous solutions containing the salts of hexacyanoiron acids inquantities of from 0.0115%, preferably 15% and polymeric acids inquantities of 0.0110%, preferably 15%, can be utilized in the presentprocess With great success.

Sometimes it is advantageous for up to 10% of organic solvents to beadded to the aqueous solutions according to the present invention. Asthe organic solvents, a wide variety of Water-miscible solvents aresuitable. Those that are preferred are, however, those which containoxygen in the molecule in the form of ether, ketone or alcohol groups;exemplary are cyclic ethers such as dioxane and tetrahydrofuran, lowerketones such as acetone and methyl ethyl ketone, and, in particular,solvents containing alcohol groups, e.g., methanol, ethanol, propanol,isopropanol, ethyleneglycol, polyethyleneglycol, glycerine,ethyleneglycol monomethylether and glycerine monoethyl ether. Themultivalent alcohols such as ethyleneglycol, glycerine and polyglycolsare particularly favorable. Also, mixtures of solvents can be used.

Further, it is often advantageous for wetting agents and/or thickeningagents to be added to the solutions of the invention. As wetting agents,alkali salts of dialkyl naphthalene sulfonic acids, such asdiisobutylnaphthalene sulfonic acid sodium salt, sulfonated fattyalcohols, fatty alcohol polyglycol ethers and glycerine are, forexample, suitable and, as thickening agents, polyvinyl alcohol,cellulose products, such as carboxymethyl cellulose or alkyl cellulose,silica gel or soluble starch products may be used.

The invention will be further illustrated by reference to the followingspecific examples:

Example I parts by weight of especially pure zinc oxide, grade A (fromthe Zinkweiss Handelsgesellschaft, Oberhausen, Germany),

80 parts by weight of Silikonharz K, a methyl silicone resin, (50%solution in toluene) (from the Wacker- Chemie, Burghausen, Germany),

parts by weight of industrial toluene,

25 parts by weight of methanol,

and 0.010.05 part by weight of Rhodamine B extra are homogenized in acolloid mill or in a high-speed rapid mixer. The suspension, now readyfor coating, is applied in known manner to a paper foil, to which a thinaluminum foil is laminated, and then dried. For the production of imageson the electrocopying material thus produced, the coating is negativelycharged by a corona discharge and then given an exposure of 5-30 secondsaccording to the reproduction scale at stop 9 in the cassette of adocument camera fitted with a reversal prism. As a light source, eight500-watt filament lamps are used. A suitable master is a line drawn withprinting on both sides.

The electrostatic image of the master thereby formed on the zinc oxidelayer is made visible by dusting over with a resin powder pigmented withcarbon black and is fixed, by heating to about 160 C., to give anirremovable copy. The resin powder used for the development consists ofthe components known as toner and carrier. As the carrier, glass ballsor iron powder are used, as in general practice; these, in conjunctionwith the toner, produce charging as a result of the triboelectriceffect, the toner acquiring the opposite charge to that of the carrier.The toner consists of a polystyrene-colophony mixture of low meltingpoint to which carbon black and, advantageously, spirit-solublenigrosine (Schultz, Farbstofi'tabellen, No. 985), as an organicdyestuff, are added. The components are melted together, the melt isground and, as a uniform grain size is advantageous for the productionof the images, the particles are separated by air sifting. For example,a fraction containing toner particles of a grain size of 2030,u is verysuitable.

For the preparation of the printing plates, the image is wiped over witha solution containing 3 parts by weight of the potassium salt of thehexacyano-iron (II) acid (K [Fe(CN 2 parts by weight of polyvinylphosphonic acid, 0.3 part by weight of diisobutyl naphthalene sulfonicacid sodium salt and 95 parts by volume of water. After brief rinsingwith water and inking up of the printing plate with greasy ink, printingcan be effected in an offset machine in the usual Way.

Example 11 180 parts by weight of zinc sulfide (a product com merciallyavailable under the name Sachtolith L),

80 parts by weight of Silikonharz K, a methyl silicone resin, (50% intoluene) (from the firm 0f Wacker, Burghausen, Germany),

160 parts by weight of a product available under the name Plexigum, apolymethacrylic acid ester in which the ester component is primarilybutyl or isobutyl, (20% in toluene),

200 parts by weight of toluene,

40 parts by weight of methanol,

and 0.01-0.05 part by weight of Bengal Rose N extra are homogenized asdescribed in Example I. This suspension is coated on an aluminum foilmade of bright rolled aluminum and is then dried. The electrocopyingmaterial prepared in this way is negatively charged with a coronadischarge and a leica diapositive is projected thereon with anillumination strength of 3 luxes, in a photographic enlarging apparatus,the exposure being of 40 seconds duration. Development is effected witha developer consisting of a toner and iron powder and fixing isaccomplished by brief heating to 100450 C.

For conversion into a printing plate, the image is wiped over with asolution containing 0.5 part by weight of the potassium salt of thehexacyano-iron (III) acid 2 parts by weight of the potassium salt of thehexacyanoiron (II) acid (K [Fe(CN) 1 part by weight of carboxymethylcellulose, 3 parts by weight of polyacrylic acid and 93.5 parts byweight of water, and it is then inked up with greasy ink.

Example III The procedure of Example I is followed but for thepreparation of a printing plate, a solution imparting the property ofwater acceptance is used which consists of 5 parts by weight of thepotassium salt of the hexacyanoiron (II) acid, 3 parts by weight ofpoly-vinyl phosphonic acid, 10 parts by volume of methanol and 83 partsby volume of water. After the fixed image has been wiped over with thesolution, the image-free parts become water conductive and the tonerimage accepts greasy ink. Prints can be made once the plate has been setup in an offset machine.

Example IV parts by weight of especially pure zinc oxide,

100 parts by Weight of a methyl silicone resin (50% solution intoluene),

parts by weight of toluene of the commercial degree of purity,

1015 parts by weight of 2,5-bis-[4-diethylamino-phenyl- (1)]-l,3,4-oxadiazole,

10 parts by weight of methanol, and

0.02 part by weight of Rhodamine B extra are together homogenized in ahigh speed mixer, after dissolving the oxadiazole in toluene. A printingplate is prepared as described in Example I, but the solution impartingthe hydrophilic properties has the following composition:

100 parts by volume of a 5% aqueous solution of the potassium salt ofhexacyano-iron (II) acid,

100 parts by volume of a 5% aqueous solution of polyvinyl phosphonicacid, and

50 parts by volume of isopropyl alcohol.

It will be obvious to those skilled in the art that many modificationsmay be made within the scope of the present invention without departingfrom the spirit thereof, and the invention includes all suchmodifications.

What is claimed is:

1. A process for the preparation of a printing plate which comprisestreating a photoelectrically conductive insulating layer, containing atleast one inorganic photosemiconductor and having a developed and fixedelectrostatic image thereon, with an aqueous solution of at least onesalt selected from the group consisting of ferrocyanides andferricyanides, in admixture with polyvinyl phosphonic acid wherebyhydrophilic properties are imparted to the image free areas of thelayer.

2. A process according to claim 1 in which the aqueous solution containsa water-miscible organic solvent.

3. A process according to claim 1 in which the insulating layer alsocontains an organic photosemiconductor.

4. A process according to claim 1 in which the insulating layer alsocontains a dyestuff sensitizer.

5. A process according to claim 1 in which the insulating layer is on asupport.

6. A process according to claim 5 in which the support is aluminum.

7. A process according to claim 5 in which the support is paper.

8. A process according to claim 1 in which the salt is an alkali metalsalt.

9. A process according to claim 1 in which the aqueous solution containsa wetting agent.

10. A process according to claim 1 in which the aqueous solutioncontains a thickener.

11. A composition adapted for treatment of photoelectrically conductiveinsulating layers comprising an aqueous solution of at least one saltselected from the group consisting of ferrocyanides and ferricyanides inadmixture with polymerized polyvinyl phosphonic acid.

12. A composition according to claim 11 containing, in addition, awater-miscible organic solvent.

13. A composition according to claim 11 in which the salt is an alkalimetal salt.

14. A composition according to claim 11 in which the aqueous solutioncontains a wetting agent.

15. A composition according to claim 11 in which the 2,988,988 aqueoussolution contains a thickener. 2,997,387 3,037,861 References Cited bythe Examiner 3,108,53 5 UNITED STATES PATENTS 5 3,211,686

2,186,945 1/1940 Wood 101149.2 2,186,946 1/1940 Wood 101-149.2

Kurz 101-1492 Tanebaum 961 Hocgl et a1 961 Uhlig 101149.2 Uber et a1.

DAVID KLEIN, Primary Examiner.

1. A PROCESS FOR THE PREPARATION OF A PRINTING PLATE WHICH COMPRISESTREATING A PHOTOELECTRICALLY CONDUCTIVE INSULATING LAYER, CONTAINING ATLEAST ONE INORGANIC PHOTOSEMICONDUCTOR AND HAVING A DEVELOPED AND FIXEDELECTROSTATIC IMAGE THEREON, WITH AN AQUEOUS SOLUTION OF AT LEAST ONESALT SELECTED FROM THE GROUP CONSISTING OF FERROCYANIDES ANDFERRICYANIDES, IN ADMIXTURE WITH POLYVINYL PHOSPHONIC ACID WHEREBYHYDROPHILIC PROPERTIES ARE IMPARTED TO THE IMAGE-FREE AREAS OF THELAYER.